The invention relates to a differential gearing unit with controllable torque and rotational speed distribution, composed of a housing, a differential gearing, a superposition gearing and an auxiliary drive, with the differential gearing comprising an input member which is driven by the vehicle engine, and two output members with output shafts, and with the superposition gearing being a planetary gear set which is connected to two members of the differential gearing and to the auxiliary drive and furthermore with the superposition gearing comprising a first and a second sun gear, a first and a second ring gear and first and second planet gears, and the first and second planet gears meshing in each case with the first and second sun gear respectively and with the first and second ring gear respectively.
A differential gearing equalizes rotational speed differences between the two output shafts, with the distribution of the torques resulting from the geometry. A differential gearing unit according to the above definition additionally makes it possible to influence the torque distribution in a targeted fashion. This is done by imposing a relatively small speed difference on the two output shafts. It is thus possible, for example in an application as an axle differential, to supply additional drive force to the faster-rotating wheel at the outside of the curve, or in an application as a central differential, to adjust the torque between the two driven axles. The driving dynamics of the vehicle are influenced in this way. This is therefore also referred to as “torque vectoring”.
Differential gearing units of said type can be arranged both as axle differentials, that is to say between the wheels of an axle, or as inter-axle differentials, that is to say between two driven axles. The additional drive force is either branched off at a suitable point upstream in the force flow or is provided by an auxiliary drive. In the latter case, it is desirable for reasons of energy balance and wear for the auxiliary drive to be at standstill when the two drive output shafts are at the same rotational speeds. In addition, in view of the driving-dynamic action of an engagement of said type, the drive force applied by the auxiliary drive must be very precisely controllable.
A differential gearing unit of said type is known from U.S. Pat. No. 5,387,161. In said axle differential unit, the superposition gearing is a single planetary gear set whose planet carrier is rotationally fixedly connected to the auxiliary drive and whose ring gear is drive-connected, by means of a fixed-axle step-up stage, to the planet carrier of the differential gearing, which is likewise embodied as a planetary gear set, and therefore to the other output shaft. A disadvantage thereof is firstly that the auxiliary drive does not act exclusively on the one output shaft. This causes complex torque ratios which make precise control of the auxiliary drive difficult. When driving in a straight line, although the auxiliary drive is at standstill, extremely high rotational speeds are generated in the coupling to the differential gearing. In addition, the auxiliary drive requires considerable engine power for an effective engagement. The overall arrangement is heavy, cumbersome and structurally complex.
WO 03/066363 A1 discloses a generic differential gearing unit which is used as an inter-axle differential and has a superposition gearing for influencing the torque distribution. Said superposition gearing is however designed with a transmission ratio nonconformity, so that, despite the two drive output shafts being at the same rotational speed, the auxiliary drive must operate in order to equalize the transmission ratio nonconformity. This results, in addition to increased wear and energy consumption, in a higher power requirement of the auxiliary drive, because the latter must output the torque which is to be introduced at a higher rotational speed.